The present invention relates to an arrangement for determining liquid and gas flow rates on the basis of the transit-time difference obtained by two measurements performed with two sound transducers spaced the same distance apart in both measurements, but with opposite directions of sound propagation. Such arrangements are known from German Pat. No. 520,484. This arrangement has the disadvantage that variations in the distances between the radiating and receiving surfaces cause measurement errors. Also, disturbing noise may result in measurement errors.
Another known arrangement is disclosed in an article by C. R. Hastings, "LE Flowmeter -- A New Device for Measuring Liquid Flow Rates," published in "Westinghouse Engineer," November 1968. Since that arrangement uses only a single fixed measuring length, and since for the measurement in the direction of flow and in the direction opposite flow, the sound transducers exchange their functions as a sound generator and a sound pickup, the error caused by distance variations in the case of two different measuring lengths is not present. That article also teaches to accommodate the sound transducers not simply inside the pipe, but also diagonally across from each other in the wall of the pipe. Thus, any adverse effects on the flow stream due to the sound transducers can be avoided. However, since the time measurements can no longer be performed simultaneously, additional circuitry is required for subtraction. In this case, the possibility of disturbances influencing the measurement result cannot be excluded.
Both the arrangement disclosed in German Pat. No. 520,484 and that disclosed in the said Hastings article are based on different methods whereby speed-of-sound variations in the medium which are caused by temperature effects, and variations in the density of the medium are eliminated. Undesired sound, however, may result in measurement errors.
Another known arrangement is disclosed in German Pat. No. 1,201,077. Since no difference method is used there, the temperature effect and variations in the density of the medium enter into the measurement result. Frequently, if not prior to each measurement, the zero must be determined anew. However, a phase detector is used to determine the transit time. The result is obtained, for example, by a cross correlation between the exciting and the received signals, this arrangement being highly insensitive to undesired sound. The output signal is a D.C. voltage change from the value 0.0 toward positive and negative values.